In recent years, lithium secondary batteries have been widely used as power supplies for driving small electronic devices and the like. Lithium secondary batteries are mainly constituted of a positive electrode, a nonaqueous electrolytic solution and a negative electrode. In particular, lithium secondary batteries constituted of a positive electrode comprised of a lithium compound oxide such as LiCoO2 and a negative electrode comprised of a carbon material or a lithium metal are used. As the nonaqueous electrolytic solution for such lithium secondary batteries, a carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) is used.
However, secondary batteries having better battery characteristics, i.e., battery cycle properties, electrical capacities and the like, are required.
In a lithium secondary battery using, for example, LiCoO2, LiMn2O4, or LiNiO2 as the positive electrode, part of a solvent in a nonaqueous electrolyte locally decomposes oxidatively during charging, and the decomposition products inhibit a desirable electrochemical reaction of the battery, thereby lowering the battery performance. This is thought to result from the electrochemical oxidation of the solvent at the interface between the positive electrode material and nonaqueous electrolytic solution.
Moreover, in a lithium secondary battery using, for example, natural graphite, artificial graphite and like highly crystallized carbon materials as the negative electrode, a solvent in a nonaqueous electrolytic solution reductively decomposes on the surface of the negative electrode during charging. Therefore, part of ethylene carbonate (EC) which is generally and widely used as a nonaqueous electrolytic solution solvent is reductively decomposed through repeated charging and discharging, thereby lowering the battery performance. Especially, propylene carbonate (PC) having a low melting point and a high dielectric constant, which is preferably used as a nonaqueous solvent since it has a high electric conductivity at low temperatures, cannot be used as a lithium secondary battery because of its very high level of decomposition.
Various suggestions have been made to improve the battery characteristics of this lithium secondary battery.
For example, Patent Document 1 discloses an electrolytic solution containing the first solvent such as propylene carbonate, the second solvent such as dimethyl carbonate, and vinylene carbonate in an amount of 0.01 to 10% by weight of a solvent mixture. Moreover, Patent Document 2 discloses an electrolytic solution containing 0.1 to 5% by weight of vinylene carbonate and asymmetrical linear carbonate.
Patent Document 3 suggests a nonaqueous solvent containing a cyclic carbonate and a linear carbonate as main ingredients and 1,3-propane sultone added thereto, and discloses the improvement of cycle property by adding 1,3-propane sultone.
Patent Document 4 discloses a nonaqueous electrolytic solution containing diketones such as ethyl levulinate and acetonylactone. Patent Document 5 discloses a nonaqueous electrolytic solution containing diketones such as acetylacetone.
Moreover, Patent Document 6 discloses a lithium secondary battery having the density of the layer of a positive electrode mixture of 3.3 to 3.7 g/cm3 and the density of the layer of a negative electrode mixture of 1.4 to 1.8 g/cm3.
As mentioned above, in known lithium secondary batteries having relatively low capacities, battery characteristics such as cycle properties are somewhat improved by adding vinylene carbonate, 1,3-propane sultone and other substances.
However, the techniques of Patent Documents 1 to 6 cannot provide satisfactory battery characteristics required for high-capacity lithium secondary batteries of these days, especially lithium secondary batteries with high-density layers of the positive electrode mixture and negative electrode mixture. An electrolytic solution having an even higher long-term cycle property is thus required.
[Patent Document 1] Japanese Unexamined Patent Publication No. H08-45545
[Patent Document 2] Japanese Unexamined Patent Publication No. H11-185806
[Patent Document 3] Japanese Unexamined Patent Publication No. 2000-3724
[Patent Document 4] Japanese Unexamined Patent Publication No. 2001-176551
[Patent Document 5] Japanese Unexamined Patent Publication No. 2001-185212
[Patent Document 6] Japanese Unexamined patent publication No. 2003-142075